The invention is directed to a salt bath based on an alkali and/or alkaline earth metal halide for the currentless production of wear resistant boride coatings on metallic workpieces at 650.degree. to 1100.degree. C. It is especially useful in the production of single phase, hard and adherent boride layers on steel to increase the wear resistance and to improve the corrosion resistance.
It has long been known to protect steel and refractory metals from wear by boriding through diffusion of the element boron into the surface of the treated workpiece and reaction with the base material there are formed dense, homogeneous layers of the respective borides, on iron, e.g. the boride FeB and Fe.sub.2 B. The borides have considerably different properties compared to the pure metals. In particular most borides are also very hard, corrosion resistant, and therewith are extremely wear resistant. The boride layers are firmly bound with the base work material through diffusion. In regard to their wear resistance, e.g. borided steels are in part superior to steels treated by nitriding or carburizing.
Therefore in the past there were developed a large number of industrial process variants according to which there can be produced boride layers, especially on steel.
In practice there was used almost exclusively boriding in solid boriding agents. Thereby the parts to be treated in iron tanks were packed in a boron supplying powder, usually mixtures of boron carbide, aluminum oxide, silicon oxide, and the like with activating additives such as ammonium fluoride or potassium borofluoride (e.g. German Pat. No. 1,796,216). The tanks were closed tightly and annealed for some time whereby the desired boride layers were built in direct solid body-solid reactions or by transport of the boron via the gas phase. This powder has a number of disadvantages.
Thus all parts must be carefully placed by hand in the powder. Furthermore, in the annealing the powder sinters strongly together so that the borided parts are very hard to take out and additionally must be subsequently purified. At the same time large amounts of boriding powder are needed which make the process extraordinarily expensive. Finally in boriding in powders many non-uniform layers must be dealt with. Quality control is not possible through judging a single part since this is not representative of the charge, since the quality of the parts depends essentially on the carefulness in placing them in the boriding powder. Small parts, parts with thin bore holes, undercuts etc., generally either cannot be borided in the powder or can only be borided with extreme expense.
Therefore, there have been numerous attempts to compensate for the disadvantages through other processes. Thus it was tried to put the boriding powder in the form of a suspension or paste on the part, evaporate the solvent and to anneal the part in the crust of boriding residues which arose, (e.g. H. Kunst. O. Schaaber, Hartereitechn. Mitt. 22 (1967), 275-284).
However, these methods which are known as paste processes are only modifications of powder boriding and have the additional disadvantage that after the treatment large amounts of persistent residues must be loosened from the parts and that it is extremely difficult to uniformly apply the paste, particularly with complicatedly shaped parts.
Likewise, it is difficult to avoid the formation of bubbles in the paste coat or the crumbling away of the crust in the annealing.
Therefore it was also tried to boride in gaseous medium, for example with a boron halide/hydrogen mixture (EPO published application No. 76488). Indeed there were obtained boride coatings. However, these are either unusable industrially or are only producible in very expensive ways. In boriding with boron halides there always occurs an uncontrollable corrosion of the base work material since the latter reacts with the boron halide with formation of metal halide and boride. Consequently there are formed porous, undercut boride coatings. Boriding with diborane is nearly impossible industrially on account of the extreme explosibility and high toxicity of this gas. Besides boriding with the mentioned gaseous media is also uneconomical because of the high price of the boron compounds. For these reasons there have been attempts to avoid the mentioned disadvantages by boriding in liquid media, especially in molten salts. Thus there have been described melts based on alkali and alkaline earth chlorides with B.sub.2 O.sub.3, borax or KBF.sub.4. However, in such melts a work material can only be borided if simultaneously there is carried out an electrolysis. Thereby the workpiece to be borided is connected cathodically, the crucible or a graphite rod serves as the anode. This process has the disadvantage that different current densities produce non-homogeneous coating thicknesses on complicated parts. Besides oxygen, chlorine or fluorine is formed at the anode as a result of which there is produced severe corrosion. Furthermore, there is difficulty in charging since an electrical contacting of the individual parts is required. For these reasons the electrolytic boriding processes in salt melts have not been introduced into the art.
In contrast only very little is known about boriding in salt melts without electrolysis. There is described in Hartereitechn, Mitt 17 [1962], 131-140 a melt made of 80% NaCl, 15% NaBF.sub.4 and 5% B.sub.4 C, in which case, however, there is simultaneously made reference to the fact that NaBF.sub.4 dissolved in the melt very quickly decomposes to NaF and BF.sub.3, which escapes. Through this instability of the melt there cannot be obtained a periodically constant boriding action. The melt is very quickly inactivated. German OS 3118585 recites a process for boriding in salt melts without electrolysis in which the boron necessary for the boriding is set free by reaction of borax with silicon carbide. Because of the oxidation of SiC to SiO.sub.2 by oxygen of the air or through decomposition of SiC with borate, however, there is very shortly formed in such melts an impervious silicate coating on the surface of the bath.
Furthermore, there are known currentless boriding salt baths which contain, in addition to boron carbide, boric acid and fluoroborate (Great Britain Pat. No. 959533) or an alkali or alkaline earth metal halide and fluoroborate (Homan U.S. Pat. No. 3,634,145). Yet these salt baths also have not been able to be put into practice.
Therefore it was the problem of the present invention to develop a salt based on alkali and/or alkaline earth metal halides for the currentless production of wear resistant boride coatings on metallic work materials at temperatures of 650.degree. to 1100.degree. C. which is simple and economical to operate, forms no crusts on the bath surface and furnishes adhering boride coatings, which especially with steels consist of (or consist essentially of), single phase Fe.sub.2 B coatings.